Abstract: A system for remotely controlling a desired door locking or other function in a vehicle or other protected environment has a transmitter and receiver for communicating a message including a sequence number, the code of a selected function to be performed and an authenticator. An algorithm in the transmitter and in the receiver has a cryptographic key and a seed code. Each algorithm generates the authenticator as a function of both the seed code and the function code; if the authenticators are equal, the message is valid. Upon each transmission the seed code is updated and the sequence number is incremented. The receiver updates its seed code according to the transmitted sequence number to keep the algorithms in synchronism.